From Norwegian patent No. 178739 it is known a method for calculating the length of consumable electrodes which are submerged in the charge of electric smelting furnaces, where the length of the electrode is calculated based on registering the weight of the electrodes. According to the Norwegian patent the weight of an electrode is being registered at constant time intervals of up till 5 minutes and where the average value for a plurality of registered weight values is calculated and plotted versus time. Thereafter a curve running thorugh the average values for a minimum period of 24 hours is calculated and this curve is being used to calculate the weight of the electrode after corrections have been made for added electrode paste and added electrode casing. Based on this, the electrode length below the electrode holder can be calculated taking into consideration the diameter and the density of the electrode.
Each registered weight value is, according to Norwegian patent No. 178739, corrected due to possible movement of the electrode during weighing. The registered weight values are further compared with preset upper and lower limits for registered weight values and registered weight values over and below the upper and lower limits are excluded.
It has been found that the method according to Norwegian patent No. 178739 does not give sufficiently accurate results, as the calculated average electrode weight for some time intervals can give a high deviation even if the registered weight values are corrected as described in the patent. This is particularly the case for deviations due to dynamic and static friction between the electrode and the furnace charge. Further varying pressure in the reaction zone in the furnace can influence the registered electrode weight.